I would be saving spirit shell in the first phase for the first of the 2 explosions and using either another spirit shell or a some kind of DR raid CD for the 2nd set of explosions. Having those explosions close together (which they shouldn't be) is usually the highest cause of deaths in my experience.
When the adds spawn I'd probably use it liberally. Depending on your assignment it might be nice to get a nice SS shield on the tanks in that phase.
Last phase I'd be using it whenever it's available.

There is four explosions per phase 1. I completely absorb the first and third using spirit shell. This means that the group should all be on full health for the following explosions which follow shortly after, so no one should be in danger of dying.

I've seen a lot of people say that they favor crit > mastery > haste for 10 mans, but every priest I look up goes mastery > crit > haste. I've been running mastery > crit > haste and it seems to work fine.

Empress is by far a disc fight. The utility of shields for large incoming damage is just too good to pass up. On that fight in particular, it'd be best to SS the explosions from the void zones on the ground. SS #1 and #3, and use the OP poh + DA for #2 and #4. As disc you could expect to see yourself very high on the meters in this fight just because of the raw aoe damage going out and disc's utility in large aoe situations.

Most of the fights favor Disc.Especially in 10man.
On empress:
On ph1
-Keep your arch for the 2nd field explosion.
-Absorb every 1st field explosion with spirit shell .
-Preshield the group in order to get multiple rapture procs before the 2nd explosion and use cascade on that as well.
On ph2
-Spirit shell the tank that is on your side.
-Pot.
Repeat ph1
On Ph3
-Remember to fear ward yourself.
-Use ss on cd.
Win.
I am crit>mastery>haste reforged and i am getting rank ones quite often(raiding heroic content mainly) if that matters anything to you.
If not i can send you 6pages of why crit would be better than mastery(No i won't do it on this thread since there are plenty mastery vs crit threads already).
Gl to whatever you choose.